A variety of mechanisms are currently available to measure the levels of fluids in containers. There are also a number of measurement techniques available for use with completely sealed and pressurized containers. According to one prior art technique, mechanical apparatus is provided by which a float, typically disposed in a transparent tube parallel to the central axis to the tank, permits direct observation of the fluid level in the tank. A disadvantage of this technique is that it requires an operator to actually observe the tank level. This can lead to incorrect level measurement due to human error.
An alternative approach utilizes a float or similar device which is directly connected to an electrical/mechanical apparatus for generating an electrical signal which represents the fluid level in the tank. This technique, however, is difficult to use or may be prohibited in the presence of explosives or flammable fluids, vapours or gases, or in tanks or containers which must be electrically isolated.
Furthermore, such prior art techniques usually require mechanical or electrical sencing devices in proximity with the fluid, resulting in service and maintenance burdens for repairing such direct contact apparatus. In certain cases, it may be necessary to shut down or delay a plant's operation in order to repair or maintain such sensing apparatus.
In the art of surveying, it is known to use the detection of phase differences between emitted and reflected light signals to measure short distances (up to approximately 10 kilometers). However, it is believed that these broad principles have not hitherto been applied to the problem of measuring fluid levels in containers.